1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer provided with a contact charging device for charging a surface of an electrostatic latent image carrier.
2. Description of the Related Art
There has been proposed a contact charging system, in which a charging brush, a charging roller or other member such as an endless belt driven to rotate is brought into contact with a surface of an electrostatic latent image carrier for charging the same.
FIG. 3 is a schematic diagram showing widths of variation of a chargeable capacity, which is caused due to variation of environmental conditions when a surface of an electrostatic latent image carrier is charged by the contact charging system. In the figure, "HH" indicates a high-temperature and high-humidity environment, "NN" indicates a normal temperature and normal humidity environment, and "LL" indicates a low-temperature and low-humidity environment. As shown in FIG. 3, the charging operation by the contact charging system is significantly affected by the variation of environmental conditions such as temperature and humidity. The chargeable capacity of the charging device significantly changes from an initial value achieved at an initial use of the charging device, when a series of operations are repeated for forming images. Particularly in the contact charging system, therefore, the above two factors cause significant variation from LL1 to HH3 (variation width X in FIG. 3) when the chargeable capacity changes from an initial value (b) to a value (a) due to copying or printing operations.
The inventors have made various studies for solving the above problem, and found that the chargeable capacity significantly varies from the initial value and will be finally stabilized within a certain width a in accordance with repetition of a series of operations for forming the images, even if the initial chargeable capacity is excessively low (b) or excessively high (c).
The reason for this can be considered as follows. Charging actions of the contact charging system can be classified into actions by discharging operation, injection charging and frictional charging. The injection charging action among them is affected by the amount of powder or particles adhering to the electrically conductive contacting member for charging the image carrier, which have adhered onto the electrically conductive member with repetitive operations of the charging device. The powder or particles may be toner, charge controlling agent, and powder shaved from the electrostatic latent image carrier. The injection charging amount is determined by the amount of the powder or particles adhering to the electrically conductive member. Therefore, the chargeable capacity significantly changes due to the repetitive printing operations (including copying operations). However, the chargeable capacity is stable when the amount of said powder or particles adhering to the electrically conductive member becomes over a specific amount.
Therefore, the inventors have completed the invention based on the finding that, if the conductive member has born an amount of impurity, which maintains the chargeable capacity within a predetermined range, from an initial state, the chargeable capacity is stabilized throughout the printing operations.